1. The Field of the Invention
The present invention relates to connectors for providing a plurality of simultaneous electrical connections to a printed circuit board by the insertion therein of an edge of the printed circuit board.
2. The Prior Art
It is well known to employ an edge connector for providing a plurality of simultaneous electrical connections to a printed circuit board.
The edge of the board is provided with an array comprising a plurality of spaced contact pad areas. The areas are in general gold plated or plated with any other corrosion-resistant substance which is suitable for providing electrical connection. The connector comprises a correspondingly spaced array of spring contacts. The edge of the board is inserted into the connector to compress the spring contacts and bring each one of the contact pads into sliding engagement with a corresponding one of the spring contacts. The spring contacts are in general also plated with gold or any other corrosion resistant electrical conductor.
The spring contacts on the connector are generally arranged in opposed pairs such that one spring contact in each pair opposes the other through the thickness of the board. The pairs are regularly spaced along the length of the edge of the board. When the board is not therebetween the opposed members of each pair, being in a non-compressed condition, approach one another closely but do not touch. Even when non-compressed there is generally some residual force pushing each spring against stops in their housing such that it requires a definite starting force to initiate the separation of a pair.
In order to insert the board between the pairs it is necessary first to separate the pairs without damage thereto. To this end the board is generally provided with chamfers on its insertable edge. The contact pads reach right to the edge of the chamfers. By the time the spring contacts have been separated by the chamfers they are in contact with the leading edges of the pads and are also exerting maximum restoring force. High attritional wear therefore occurs where the pads meet the chamfers. Having been inserted to the ends of the chamfers, the board is further introduced between the pairs of spring contacts until each spring contact lies centrally on a corresponding one of the connector pads on the board. During this further introduction the pairs exert maximum restoring opposing forces against one another causing considerable and undesirable attritional wear as each spring contact slides against its pad. Plastic flow of the non-corroding electrically conducting layer can also occur.
The attrition of the surfaces of both the spring contacts and the pads causes the exposure to air of the underlying non-noble substrate which then becomes liable to atmospheric oxidation bringing about a degradation of the electrical conductivity of the spring contact to pad interface.
The plastic flow of the corrosion resistant layers renders their thickness less along the path of sliding contact between each spring contact and its corresponding pad. The layers are therefore subject to a greater susceptibilty to disappearance by attrition.
In the presence of possible oxidation the reliability of an edge connector decreases rapidly. When there are plural sets of contacts the chances of all sets simultaneously providing satisfactory operation is unacceptably small.
It has been one solution to the above problem to offset the effects of oxidation by arranging that the force between opposing pairs of spring contacts is very large indeed so that any oxide layer is broken up and the heat generated by friction causes small areas of air-excluding spring-to-pad welds. This approach causes the insertion force of a board into a connector to be extremely large and the withdrawal force to be correspondingly great. The rate of wear of the spring contacts and pads is enormous so that only a limited number of insertions and withdrawals is possible before a board and connector combination has to be discarded. The boards and connector housings must be unnecessarily mechanically strong to withstand the forces exerted thereon.
It has been another approach to provide that separate means is provided for lowering the spring contacts onto the board by the use of manually operable levers and the like. This approach relieves attritional wear but substitutes complexity and high cost, the connectors themselves taking up much space. Consequently, although successful, this approach is reserved for test equipment for testing boards.
It is therefore desirable to provide an edge connector of simple construction and small size for the insertion therein of a board for the establishing of electrical connection therebetween with substantially no sliding between spring contacts in the connector and contact pad areas on the board.